It's a polo shirt, which is a smarter type of tee-shirt
In the UK a "shirt" is typically non-stretch, buttons down the front and usually has a collar (unless it's a grandad or mandarin type) and is the most formal.
Long sleeves more formal than short
A polo shirt is more smart than a tee-shirt because its usually less stretchy, has a couple of buttons and a collar.
T-shirt....stretch, no buttons, round or v-neck, casual