A First grade is usually anything that has scored 70% or above. This is generally the highest level of award, as well as the hardest one to achieve, that you can get at an undergraduate level.
A 2:1 grade is usually anything that has scored 60-69%. Again, this is what most students aim for, and is one of the most common grades to get. A 2:1 is the upper-middle grade on the scale.
A 2:2 grade is anything that scores 50-59%. This is the next grade down from a 2:1, and is still considered a decent grade to achieve. Many students worry about coming out with a 2:2 degree overall, but there is plenty you can do with a 2:2 degree.
A third grad us anything that scores 40-49% A third is the lowest class of degree that you can get and pass with.
A fail grade is anything under 40%. You will not get a classification under 40%. You may have to resit any assignments or exams that you have failed, in order to pass your degree or year.