A beat is made of multiple instruments and a song usually has multiple vocal layers. The volume of each layer has to be adjusted in order to make the full song sound ”in balance”. You don’t want anything to be too loud or too soft compared to the rest of the mix. 

Panning means adjusting the positions of a vocal track or instrument in the stereo field. It helps to make a song element stand out and be heard clearly without ”clashing” with another vocal track or instrument. It's also used to create a wide stereo image and room in the mix.

Equalizing means adjusting parts of the audio frequencies to your liking. So for each instrument or vocal track, you can boost or lower certain frequencies to create a nice balance throughout the frequency spectrum of a mix.

The final fundamental ingredient to every professional mix is compression. With compression it is possible to control the dynamic range of a track. Especially the human voice is a very dynamic instrument and in order to be received well throughout a song or even stand upfront or "in your face", compression is a vital step.

Additional effects such as reverbs, delays, filters or distortion can be used to add character and space to the mix. The amount in which the effect is applied can also be ”automated” and adjusted during the song. For example in a break you might want the vocal reverb to last longer than during the beat.

Mastering is the term used for the process of preparing your audio mix for distribution. Mastering involves tiny adjustments in the master equalization, compression, limiting and more. The process will boost the volume of your audio without disturbing the balance. Balancing issues and enhancing audio characteristics is the main goal.