180天停留期是什么意思？

the visa itself will say very clearly about it
either 'maximum stay 180 days a time' or 'maximum stay 180 days in any calendar year' or something like that

这两个说法是一致的。每个自然年不可以待超过180天。当然一次也不可以连续待超过180天。

Visit visas allow stays of no longer than 6 months (or 180 days) per visit.
https://www.gov.uk/government/publications/entry-clearance-visas-by-length/entry-clearance-visas-by-length

具体看哪个国家的政策，都有的

连续的12个月里面最多停留不可超过180天。