we'll is the short form of 'we will' — and sometimes of 'we shall', though almost nobody uses 'shall' this way now outside of formal British English. It is used for the future: what we are going to do, what we predict, what we promise or agree to. 'We'll call you tomorrow.' 'We'll be late.' 'We'll see.' In speech, English speakers use the short form nearly all the time; saying 'we will' in full puts heavy stress on the will, which makes it sound like insistence or a firm promise — 'We WILL finish this today.' So the short form is normal and the long form is the marked one. In writing, use we'll in emails, messages and anything friendly; keep 'we will' for formal writing like reports and contracts. Say it as one syllable, /wiːl/ — it sounds the same as 'wheel'.