Latency means how much lag you will experience while playing. «ms» stands for milliseconds. 1000 ms is 1 second.
So while choosing a server you can look at the numbers.
25 ms is pretty fast. you won’t experience any lag.
100-250 ms is a bit slower but still an enjoyable experience to play.
250-400 ms is a bit laggy. sometimes you have to wait a bit to do stuff like eating.
400-1000 ms is hard to play. the map and objects have some loading time till you can interact with them.
so choose wisely 
your character might die, if you can’t eat.
(the image shows the latency from europe. if you are located somewhere else, the numbers will be completly different.)