Outside of Israel (or at least in America), a “chaser” is what you’d drink after doing a shot of something. It’s the main drink you’re drinking, while the shot is something that is over quickly. For example, people like to drink beer after doing a shot of tequila. To be clear — a shot is the hard alcohol drank from a small glass quickly, in one gulp.
In Israel, however, the phrases are completely reversed in meaning. Here, a chaser is the alcohol drank in one quick gulp and it comes alongside the main drink (i.e. beer). Very strange. Furthermore, the difference between “chaser” and “shot” is merely a matter of size. Because Israel doesn’t really have a drinking culture. I suppose they had to invent a new word to mean “half of a shot.” 
So when you’re in a bar, remember “chasers” are small shots that come alongside your main drink and “shots” are just double the size of chasers .
Zabaj: the Junk in Israel's Trunk 