There are two ways of being able to measure muscular strength. 

Absolute strength is considered the total  amount of load or force a person can lift. This is regardless of their body weight. 

• It is measured in raw metric numbers, such as benching 275 lbs. in a bench press or squatting 405 lbs.
• This matters for those who emphasis tracking weights for methods like progressive overload. It can also be of high importance for those who lift for sport. A power lifter or strongman is judged off of his absolute strength in comparison to his competitors. 

Relative strength is strength in relation to body weight. For example, how strong are you per pound of body weight. 

• This is calculated as absolute strength divided by body weight! For example, if I weigh 185 lbs. & squat 405 lbs., my relative strength is 2.19. 
• This can be highly important to improve if you are competitive in sports that require a lot of relative strength, such as: rock climbing, wrestling, gymnastics, combat sports & martial arts, etc.

Why might these be important to understand for weightlifting or trying to gain muscle?

Understand how to tailor your programs

• If you are aiming to maximize muscle growth, prioritizing absolute strength will allow you to lift heavier loads & progressive overload efficiently enough to promote hypertrophy. 
• Staying powerful & lean is when something like relative strength comes into play. No need to worry about excessive weight gain, & it is highly optimal for those sports I mentioned above. 

Body composition

• Excessive weight gain can reduce relative strength. This will impact performance in body-weight dependent exercises or activities. 
• Ensure to balance muscle gain & body weight if this is something you are worried about & want to ensure that you are optimizing performance. 

Optimizing overload

• Progressively tracking both relative & absolute strength will help ensure consistent progress. 

Sport performance

• Athletes in weight-class sports (e.g., MMA, wrestling) must maximize relative strength to be as strong as possible without moving up a weight class.
• Powerlifters and strongmen focus on absolute strength to lift the most weight possible, regardless of body weight.

Longevity

• Prioritizing relative strength helps avoid unnecessary weight gain that can stress joints.
• Training absolute strength ensures overall muscle development and resilience.

Peer review articles: 

Absolute Vs. Relative Machine Strength as Predictors of Function in Older Adults

https://journals.lww.com/nsca-jscr/abstract/2002/11000/Absolute_Vs__Relative_Machine_Strength_as.23.aspx

On the relative and absolute strength of a memory trace

https://link.springer.com/article/10.3758/BF03200880

Differences of Relative and Absolute Strength of Individuals With Spinal Cord Injury From Able-Bodied Subjects: A Discriminant Analysis

https://journals.humankinetics.com/view/journals/jsr/28/7/article-p699.xml?alreadyAuthRedirecting

Relative and absolute strength of response as a function of frequency of reinforcement

https://pmc.ncbi.nlm.nih.gov/articles/PMC1404074/

Relationships Between Absolute and Relative Strength and Power in Male Police Officers of Varying Strength Levels

https://link.springer.com/article/10.1007/s42978-019-00033-5

Relative Strength, but Not Absolute Muscle Strength, Is Higher in Exercising Compared to Non-Exercising Older Women

https://www.mdpi.com/2075-4663/7/1/19

Absolute and relative maximum strength measures show differences in their correlations with sprint and jump performances in trained youth soccer players

https://eprints.qut.edu.au/254018/

Using Absolute and Relative Muscle Endurance to Estimate Maximal Strength in Young Athletes

https://journals.humankinetics.com/view/journals/ijspp/2/3/article-p305.xml