Gunpla refers to Gundam Plastic Models, Gundams being the large mechs depicted in the long running Japanese cartoon series. The models themselves range in size and in build difficulty. Every model has a specific Scale and Grade.
The two most common sizes are 1/144 scale and 1/100 scale models. 1/144s range from about 5 to 6 inches tall, while 1/100s are about 7 to 8 inches tall. There are also a few other scales like 1/60 and 1/72 which will obviously be much large then the 1/144s and 1/100s.
As far as grades go there are four main ones. The first is No Grade. These models are really inexpensive. However, although you may be able to get them for under ten bucks you really sacrifice articulation and detail. Some people like No Grades, but I'm not a big fan.
Most of my collection is made up of High Grades
High Grades have a good amount of detail and most have good if not great articulation. Another selling point for the high grades are their price. You can easily find a sweet HG 1/144 and some 1/100 kits for under twenty bucks (plus shipping if bought online).
Next up are Master Grades
Lastly we have Perfect Grades