Frequency is the last of the three terms, originally mentioned in the What is Volume? - Explained article. If you have not read the other articles, I would suggest reading those first. They can be found here:
What is Volume? - Explained
What is Intensity? - Explained
Volume and Intensity often get the spotlight and leave little brother frequency in the back seat. You could say that they are more important than frequency. A lot of people do. But frequency plays an important role. Frequency governs volume and intensity. Frequency is the way you organize your volume and intensity over a given period of time.
There are three things to consider for frequency: Learning, recovery, and organization (life).
As we discussed previously, strength is a skill. There are neurological adaptations that take place as you practice a certain lift more and more. Think of studying for a test. I know there are times that you've crammed for a test. We've all done it. You stay up the night before and do all the studying at once. You may do okay on the test but you certainly don't remember any of it a couple of weeks down the road. It makes much more sense to spread the studying out over the course of the week before the test. It's the same with training. We COULD do all of our weekly training volume in one day but we would certainly not learn to be proficient at the movements because as we get tired, our form breaks down and we aren't performing the movements correctly.
Weight training is a physically demanding activity. We must allow our bodies the appropriate amount of time to recover. There is a large body of evidence suggesting that splitting up your total volume for the week across multiple sessions is better than doing all your volume in one session for neuromuscular adaptations, hormonal markers for recovery, strength improvements, and muscle hypertrophy, although this may seem obvious to some.
Raastad and colleagues studied the Norwegian power lifting team. They were divided into two groups. One group trained 6 sessions per week and the other trained 3 sessions per week. The 6 session group did half as many sets per session so that each group performed the same number of sets. Intensity was also equated. The 6 session per week group had more hypertrophy and strength gains. 
Organization is simply fitting the training into your life. How many times can you train per week? Maybe you can only train 3 times per week. Maybe you have a home gym and can train any day of the week. Personally, I travel for work 1-2 days per week so 6 sessions per week would be unmanageable for me.
Frequency recommendations are a little less scientific than strength or volume recommendations. My recommendation is to take what you have determined to be your needed volume per week, determine how many days per week you have available to train and split your volume amongst those training sessions. If you find you have one day loaded too heavy or a day that is too short, you can reshuffle the sessions as needed.
Thanks for reading! I hope this article was helpful to you. Don't forget to like, share, and comment on social media. It helps a ton. As always, God bless you and your family and I'll see you next time.
 Raastad, T., et al., Powerlifters improved strength and muscular adaptations to a
greater extent when equal total training volume was divided into 6 compared to
3 training sessions per week, in 17th annual conference of the ECSS, Brugge 4-7 2012.