There were 613 laws in the Old Testament - laws God put in place. Over the years, the Pharisees and religious leaders added many more - what they called oral traditions. They imposed all of these on the people. This is legalism multiplied by a hundred. Imagine the burdens the people carried... they wanted to be a good Jew. They wanted to please God, but keeping all these laws was near to impossible. They were tired and worn out. I wonder if Jesus saw this struggle in their spirits when He spoke these words.
Rest can be traced all the way back to the creation: God rested on the 7th day. And God also commanded in the OT that the Sabbath be a day of rest. We know that rest for our physical bodies has GREAT benefits: improved health, enhanced mood, increased productivity, and mental clarity which helps us make better decisions. Resting heals. No wonder our bodies need rest.
But our spirits need rest too. We need spiritual rest, and I believe that is what Jesus is referring to in these words. Without it, we are spiritually weak and too worn out to accomplish what God has put before us. Oh yes, we may still be working and ministering but also feeling worn out and fatigued. Drained. spent. If you're feeling this way, most likely it's because you haven't taken the time to rest in Jesus.
Let me suggest that when you enter His presence (and I hope you do every day), just sit and let Him minister to you. Don't talk. Just sit. Read Scripture. We often come with our minds full. But when we rest in Him, He empties those minds out of all that stuff and replaces it with His resting promises. Picture yourself sitting on His lap, head against His chest, listening to His heartbeat. That's TRUE rest. We need this every day. My prayer is that each of us will come and REST today. Hope this encourages you today.