When we shorten words we use apostrophes to show that letters have been missed out.
Sometimes the same shortened form can stand for more than one full version.
She would come if she could. / She'd come if she could.
She had been to feed the ducks in the park. / She'd been to feed the ducks in the park.
We use 'd to shorten both the words would and had, but people can understand what we mean when they read the whole sentence.
Another shortened form that stands for more than one full version is 's.
It is Monday today. / It's Monday today.
It has been raining all morning. / It's been raining all morning.