It's basically an alternative to A levels. You either do A levels (usually 3 or 4 in whichever subjects you want) or international baccalaureate (6 subjects which have to include English, maths, a science, a language, a humanities subject, and one other). It's an international award, whereas A levels are just done in the UK. IB is generally considered harder than A levels due to you having to be consistently good across a range of subjects, but there's no discrimination for/against either award.