What is a land creature?
A permanent which is both a land and a creature.

Let's together find out what they exactly mean with 'land creatures'.
In the Comprehensive Rules we find the following regarding multiple card type:


  205.2b Some objects have more than one card type (for example, an artifact creature). Such objects satisfy the criteria for any effect
  that applies to any of their card types.


Meaning that in this example the object benefit from being an artifact as well as a creature. But on the flip side..


  300.2. Some objects have more than one card type (for example, an artifact creature). Such objects combine the aspects of each of those
  card types, and are subject to spells and abilities that affect either
  or all of those card types


So they are also approachable by all the types the card currently has.

But we want to know what cards are being affected by Halimar Tidecaller static ability ("Land creatures you control have flying."). Does that mean all lands or all creatures or all lands that are also creatures?

It says on the card 'Land creatures you control' which indicate that any object that satisfy that criteria is affected. So not just lands or not just creatures, but only 'land creatures'.