A Work visa is a type of visa required to work in another country.
Some countries have visa-free work rights. For example, EU citizens have the right to work in other EU countries. Although if you're British, your EU citizenship may soon be stripped from you due to the disastrous Brexit political onmishambles, leaving work rights to be decided upon by the local jurisdiction.
Other countries have different rules, which may differ between jurisdictions (e.g. provinces within a country). It may be easier to get a visa to teach English if you're a teacher from countries like the UK, USA, Canada, the Republic of Ireland, Australia or New Zealand. South Africans are classed as "native speakers" in some jurisdictions, but not others, regardless of whether or not they are actually an English native speaker. The Chinese notably have a tendency to keep arbitrarily changing the rules without really telling anyone, which can lead to multiple visa applications being rejected.