She'll is how we write and say she will when we are talking at normal speed. The two words squeeze together: she + will → she'll. In older or very formal English it can also stand for she shall, but in the English you actually hear it is she will almost every time — plans, promises, guesses about the future: She'll be late. She'll help you. This is the everyday form. In careful school writing, teachers sometimes want the full she will; in speech, she'll is the natural one. The apostrophe is doing a job: it shows letters have dropped out. Though actually, on a quick glance it can look like shell (the hard cover of an egg or a sea creature). That is a different word.