Guild Wars 2

In GW2 WvW Leaderboard (https://leaderboards.guildwars2.com/en/eu/wvw) Seafarer’s Rest realm is the best in WvW. It has Tier 3. But 2nd and 3rd best currently are Vizunah Square [FR] and Piken Square. They have Tier 1.
What do these Tiers mean?

in this case it simply refers to the current match.

ie: SR is playing against the other two servers listed as being ‘tier 3’

Nowadays, the tiers do not mean as much as they used to. When Guild Wars 2 first came out you played against the tier you played in. If you did better then you would eventually work up to anothe tier until you got into tier 2 and then 1.

tier 1 has the top 3 servers playing one another which are the servers that care about WvW the most.