ही can be used to mean “only” and “just” :

Only I work here – यहाँ मैं ही काम करता हूँ  

यहाँ = here. मैं = I. ही = only. काम करता = work. हूँ  = suffix for first person

When ही is used after pronouns, it can give the sentence an emphatic meaning :

They themselves will come – वे आप ही आएँगे

वे आप = themselves. ही = emphatic particle. आएँगे = will come.

When आप and ही are put together, it can give the sentence the meaning, “on one’s own” or “of one’s accord” :

He went to the hospital on his own – वह आप ही अस्पताल गया

वह = he. आप ही = on his own. अस्पताल = hospital. गया = went

There are certain pronunciation changes that occur when ही is combined with other words :

When demonstratives like “this” and “that” the ह sound may undergo elision :

इस (this) + ही = इसी (this very)

The ह sound may be dropped when preceded by pronouns like वह :

वह (he) +  ही = वही (he himself)

With certain words, the final vowel may be nasalized with ही :

कहाँ +  ही = कहीं (somewhere)