Herd immunity as per google is “the resistance to the spread of a contagious disease within a population that results if a sufficiently high proportion of individuals are immune to the disease, especially through vaccination”. Put simply, herd immunity represents the tipping point where the infection cannot effectively spread further.
An analogy might help clear this up. Imagine you’re on an island somewhere and you are surrounded by driftwood, sticks, and dried up newspaper. At some point you decide to get a fire going. The fire is COVID-19. At first you don’t build a fire pit, and unsurprisingly the fire spread rapidly throughout the island. So, you rewind the clock, and this time you create a fire pit before you light the fire by finding some rocks and closing off an area that will become the fire pit. That is effectively what physical distancing does to contain COVID-19. But of course, you’re on this imaginary island full of flammable stuff, so eventually an ember flies outside of the fire pit and sparks another fire. As you can imagine this can keep happening.
So how do you prevent more fires from forming? Well if you somehow protected the nearby driftwood/sticks/newspaper then it could help prevent new fires from forming. One way to do that would be to spray water [vaccine] on the surrounding driftwood/sticks/newspaper. Of course, you don’t have to do that to 100% of the island. You just need to do so on a sufficient amount so that it is unlikely more fires will start. The percentage of the island that needs to be doused in water to protect the rest of the island represents “herd immunity”. Maybe for this island you have to douse 70% of the island because it is so full of flammable stuff, whereas on another island that was mainly beach the % would much lower to achieve “herd immunity”.
Back to real-life, the magic number that provides herd immunity varies based on the infection and how easily it can spread. We don’t know what it will be for COVID-19, but probably around 60% of the population. Thus “herd immunity” will likely occur once about 50% of the population is immune.
Ok so how do you “get immune”? Either by being exposed to the virus and fighting it off, or by getting vaccinated. Ok, so why don’t we just let the virus run its course so that enough of the population is exposed? A few countries tried that approach, and it did not work out very well.
Hope that helps to clear up what we mean when we refer to “herd immunity”. If you are still confused, here’s a good video on it https://www.youtube.com/watch?v=cEn1PKyBUNc.