A 'base tan' refers to building some color before a big sun event (like a beach vacation) on the theory that it'll protect against burning once there — the protection it actually provides is real but much smaller than commonly believed.
The protection is minimal
A developed tan provides roughly the UV-blocking equivalent of a very low SPF (often cited around SPF 3-4) — nowhere near enough to skip proper sunscreen at a sunnier destination.
The real cost of building it
Building a base tan means deliberately accumulating UV exposure and skin damage beforehand — the marginal protection gained is generally considered not worth the additional cumulative UV exposure by most dermatological guidance.
A better alternative
Traveling with proper SPF, understanding the destination's typical UV Index, and pacing initial exposure carefully achieves the actual goal (avoiding a vacation burn) without the tradeoff of extra pre-trip UV damage.
Do dermatologists recommend base tanning before a trip?
Generally no — the modest protection a base tan provides is usually considered not worth the added UV exposure, and proper SPF plus gradual, UV-aware exposure at the destination is the more commonly recommended approach.