so every thing that u see around you exist in a specific wave form, apart from their physical state. This is called the natural frequency. Resonance occurs when an external wave meets an existing wave and they are in-sync. meaning that the oscillation occurs exactly the same for the two merging waves. both the minimum peak and maximum peak are aligned.
The simplest way to imagine this is when you are pushing a swing when there is someone on it, if you push the swing away as it is coming towards you, you will only slow the swing down. If you push the swing at random time or random position, the swing will have its speed change randomly (increase or decrease)
However, if you wait at the very end of the swinging motion, and push it away as soon as the swing begins to swing away from you, you will at every single time add more force to the swinging motion, hence, it will constantly be at a higher state of motion (vibration). your interaction with the swing is in-sync.