Tor: What are obfs4, Snowflake, and WebTunnel?

Short answer

These are Tor circumvention transports that make connecting to the network possible under different kinds of blocking. obfs4 commonly uses unpublished bridges, while Snowflake and WebTunnel use different approaches. No transport works on every network. Try the options offered in Tor Browser's connection settings and consult the current Tor documentation, since availability and effectiveness can change.

Sources

Tor Project: Using bridges, Tor Project: First launch.