When we talk about making up for something, we usually mean trying to compensate for a deficiency or weakness. For example, if you're not very good at math, you might try to make up for it by studying extra hard. Or if you feel like you didn't spend enough time with your partner last week, you might try to make up for it by planning a special date night.
In some cases, making up for something can simply mean doing something nice or helpful after you've done something that was hurtful or thoughtless. For instance, if you forget your friend's birthday, you might try to make up for it by buying them a really great present.
Generally speaking, when we make an effort to make up for something, we are trying to rectify a situation where we feel like we have fallen short in some way. Whether that means working harder, being more thoughtful or just taking some extra time to show our loved ones how much they mean to us, making up for lost ground is always a worthwhile endeavor.