A bullrun is a short period when prices rise rapidly. By the way, this is true not only for crypto but also for stocks, for example. A group of investors gets more confident and starts buying more as a result. This group of people are called "Bulls. This causes the price to rise and this in turn attracts new investors, causing the price to rise even further. So this puts the price in a positive feedback loop.

Bullruns from the past

In 2017, the crypto world experienced a massive bull run. If we look at the beginning of the year 2017, the price of a bitcoin was around $1,000.00.
If we look at the beginning of mid-November of 2017, we see that on November 15, the price was at $6500.00. A month later on, December 18, the price of a bitcoin touched $19,800.
This is an increase of over 300%. During this gigantic (but also short) bullrun, a lot of people decided to get in, driving the price even higher.