Properties which are the same in all directions are isotropic. For example, most liquids and materials composed of small randomly oriented crystals are isotropic in all properties while crystalline materials are, in general, anisotropic.
An isotropic signal is a signal transmitted with equal strength in every direction in space. The energy required to broadcast an isotropic signal detectable over interstellar, or intergalactic, distances would be vastly greater than that needed to send a signal of equivalent strength from a directional antenna. Only a Type II or Type III Kardashev civilization would be capable of transmitting an easily detectable message in this way.
Anisotropy is a variation in a property with direction. The cosmic microwave background, for example, shows a slight anisotropy in temperature of a few microkelvins from one tiny patch of sky to another.