One stitch per square
Aida is woven so the holes form an easy-to-see grid. Each cross stitch covers one square, so the fabric count is also the number of stitches per inch: 14-count Aida gives 14 stitches per inch. Many cross-stitch patterns are designed with 14-count Aida in mind.
Count changes the finished size
The same pattern gets smaller as the count goes up. A design 70 stitches wide is 5 inches across on 14-count Aida (70 ÷ 14) and about 3.9 inches on 18-count (70 ÷ 18).
Lower counts have bigger squares, which usually need more strands of floss to cover them.