@What does shows the doer mean

The verb hasati हसति in the sentence

hasati vipraH हसति विप्रः "the priest is laughing"

ends in /tip because the priest is singular (not priests) and it is third person (not you and not me). But when many priests do the action the verb has /jhi —

hasanti viprAH हसन्ति विप्राः "the priests are laughing"

And when the doer is first person singular, ending is /mip —

hasAmi viprAH हसामि विप्राः "I, the priest, am laughing"

So we use different endings depending on the number and person of the doer. All these endings are said to be showing the doer.

So, are you saying that all verbs change their endings according to how many doers they have?

No. I am saying that some verbs do that, like these hasati हसति hasanti हसन्ति. Others do not do it. The ones that do it are said to show the doer.

Please show an example of some verbs that do not show the doer.

Here we have the verbs **dRzyate दृश्यते **dRzyete दृश्येते and **dRzyante दृश्यन्ते —

vipreNa hayo dRzyate विप्रेण हयो दृश्यते "the priest sees the horse", "the horse is seen by the priest"

vipreNa hayau dRzyete विप्रेण हयौ दृश्येते "the priest sees the 2 horses", "the 2 horses are seen by the priest"

vipreNa hayA dRzyante विप्रेण हया दृश्यन्ते "the priest sees the 3+ horses"

vipreNa hayo dRzye विप्रेण हयो दृश्ये "the priest sees that I am a horse", "I, a horse, am seen by the priest"

Here the endings on the verbs show the number and person of the horses. Yet, the horses are not doers here, they are objects. So these show the object. They do not show the doer — the doer is the priest, here.