Let’s begin with an easy example: biceps vs triceps. When you strengthen the biceps, you are stretching the triceps. These two muscle groups work together but in opposite directions. A biceps curl puts the biceps in a shortened position while the triceps muscle is lengthened. For maximum strength, mechanics and balance, both muscle groups need to be strengthened equally. This is especially important for male weightlifters because you may prefer to strengthen the front of your body (biceps, chest, abs, etc).
Here are a few more examples of opposite muscle groups: abs vs. back extensors, quads vs hamstrings, hip abductors vs hip adductors, hip flexors vs hip extensors, pecs vs upper back. No matter what muscle group you are working on, there is an opposite group that needs equal attention. The easiest way to do so is to do the same number of exercises for the opposing groups. If you are doing 3 sets of 10 biceps curls, you also should do 3 sets of 10 triceps extensions.
The opposite muscle groups do not have to be strengthened on the same day, but I would recommend it for simplicity or if you have trouble remembering which muscles you worked each day. Typically, you will divide up your weight training into body parts for a weekly schedule, so here is an example of how to train opposite muscle groups on the same days.
Monday: chest and upper back
Tuesday: legs (all groups)
Wednesday: shoulders and arms (front and back)
Thursday: abs and low back
If you have more time per session and want to work each muscle group several times/week, try this:
Monday: chest, upper back, shoulders, arms
Tuesday: legs, abs, low back
Wednesday: chest, upper back, shoulders, arms
Thursday: legs, abs, low back