The radiation pattern of an antenna indicates the power radiated (or received) by the antenna towards/from a specific direction.
To understand this, you can make an analogy with different light sources.
An incandescent light bulb hanging from a wire emits light equally in all directions (except the bulb socket itself). The equivalent antenna would be an isotropic antenna.
A fluorescent tube emits light all around, but not to its axis. This corresponds with omnidirectional antennas, which radiate in all directions except the direction of its axis.
This is the radiation pattern of a typical omni antenna supplied as standard with video transmitters and receivers:
A flashlight, that uses a parabolic reflector, concentrates all the light in one direction, which we call the beam of the flashlight. The equivalent antenna is a directional antenna that concentrates the radiated energy into a region of space, as in the following example: