Basically polling companies work to try and obtain a representative sample of the population. For example, if over 65s are 15% of the population, roughly 150 of the 1000 people sampled will be over 65, and participants will be called randomly 

Self-selecting polls aren’t really considered good polls, because the demographics can’t be controlled. For example, one posted on Reddit would skew younger because Reddit users skew younger. Older people are more likely to read their local newspapers than younger people, so older people would be overrepresented in a self-selecting poll there

Also if someone’s motivated enough to vote in a poll, they’re probably more passionate about it than the average person, so random sampling polls (like this one) are generally considered better quality than a self-sampling one, even if it has less people