The word "卫生" can be a noun or adjective and means something like health, hygiene, or sanitary, hygienic respectively. The character "卫", to guard, to protect, to defend.

The complete last sentence is:

"Yes. But some roadside stalls are not very hygienic. Try to pick clean ones."

the 卫 in 卫生 has the pinyin wèi. The falling tone mark is the 4th tone out of Mandarin's 4 tones. So, they probably meant to write "we4i" as "wei4", with the 4 signifying the tone.