According to the Merriam Webster dictionary effort (noun) is: work done by the mind or body : energy used to do something, a serious attempt to do something, something produced by work.
Effort is a reflection of interest. You can never force someone to see that you're "worth it." You can never force someone to see your value. You can never force someone to treat you how you deserve to be treated.
If someone wants you to be around, they'll make an effort to put you there, they won't make an excuse. You keep those you love around because you want them to be there. If you aren't putting out any effort to keep them close to you, don't be surprised when they get up and walk away.
I'm the type of person that if I have to ask for your attention, I don't want it. I'm completely fine by myself because I know what I bring to the table. There is no reason to chase someone, because if they've started distancing themselves, you know where you stand.
Sure, sometimes life gets busy but that's where this E-word comes in. It's a matter of priorities. If someone or something isn't one of your priorities, your effort will most certainly show that.
In relationships, if the feelings are mutual, the effort will be equal. If you feel like there is something one-sided going on, you're generally not one of their priorities. There is no need to chase someone or wait around for them to put effort in. Chances are, they won't.
Effort isn't only in relationships though. You have to put effort into whatever you want out of life. If you're wanting to lose weight, you have to put effort into dieting and exercise. If you want a new job, you have to actively seek one out. No matter what you want or what your goal is, if you don't put any effort towards it, you will never see it happen. You get out what you put in.
You can't kinda try to do something and hope to be the best at it. You can't kinda pursue someone and expect them to stay around. I'm the type of person where if I feel like I'm put on the back burner, I lose interest. There are so many other people like this.