Jelly rolls contain 40 stripes of co-ordinating fabric which measure 2½ in x 45 in. They are individual strips (not joined together) and you have just over 2½ metres of fabric in one roll.
If you find it hard to put colours and fabrics together, then jelly rolls are great because the thinking has been done for you. All the fabrics in a jelly roll are designed to co-ordinate with one another. If you have fallen in love with a particular fabric collection, then you can have a piece of each without the expense or buying a fat quarter of each, which would cost a lot more.
As they are already cut at 2½ in, it also saves you time.
(For people in the UK, jelly roll is the name given to what we know as swiss roll!)