Aida count refers to how many squares per inch the fabric has. 14 count Aida has 14 squares per inch. 18 count has 18 squares per inch.
Higher count means smaller squares, which means smaller stitches and finer detail. Lower count means bigger squares, which is easier for beginners.
14 count is the most popular for general cross stitch. 11 count is good for beginners or large projects. 16 and 18 count are for more detailed work.
The count directly affects the finished size of your project. 100 stitches on 14 count Aida measures about 7.1 inches. The same 100 stitches on 18 count measures about 5.6 inches.