Weedle is a bug and a poison type pokemon. Obviously, it is a yellow caterpillar. Ash wasn't able to catch this one because he was distracted by another trainer when he was about to catch it.
In Pokemon Go, It can be obtained as well by hatching from a 2 km egg. If used as a buddy, it needs 1 km distance to get one Weedle candy. Its max CP is at 397 (four points higher than Caterpie). It can learn Bug Bite and Poison Sting as fast moves while Struggle is its ONLY charge moves.
It is just a bug type so fairly weak. This is still caught for Pokedex entry. As a bug type Pokemon, it is weak against flying, fire and pyschic Pokemon. It is super strong, however, against fighting, bug and fairy types. Just like Caterpie, it is very useful especially when you are trying to increase your XP for mass evolution. It costs only 12 weedle candies to evolve to the second level and you can catch it in parks, roads, even lakes, so pretty much everywhere.