～てくれてありがとうThank you for ~ ing

These て-form patterns are opposites. てすみません expresses apology for your action, e.g., 遅れてすみません ('Sorry for being late.'), while てくれてありがとう expresses gratitude for someone else's action, e.g., 来てくれてありがとう ('Thank you for coming.'), implying that the person's coming benefitted you.

くれる is a basic verb meaning 'to give,' usually used when someone from outside the speaker's group gives something to the speaker or their in-group. For example, プレゼントをくれた means '(someone) gave me a present.' It also appears in the pattern てくれてありがとう to thank someone for a favor. For example, 日本語を教えてくれてありがとう means 'Thank you for teaching me Japanese.'

てくれる describes someone doing something that benefits you, like 来てくれた ('He/you/they came for me.'), implying that their coming helped you. てくれてありがとう specifically expresses gratitude for the action, e.g. 来てくれてありがとう ('Thank you for coming.'). The base meaning is the same, but one contains an explicit message of thanks.

The Verb + て form connects verbs or parts of a sentence. A common use is to express thanks by combining it with the giving verb くれる (also in the Verb + て form) and ありがとう. For example, 教えてくれてありがとう means 'Thank you for teaching me.' This is a casual form, and a more polite version would use honorific verbs (くださる or いただく) and add ございます.