I'm pretty sure those IDs are created by Sigil when you use the "Generate Table of Contents" function. The first link - if it is near the top of the page simply links to the page itself. If you have more links on the page, or it's further down and not at the top, then the link needs an ID to point to. If you don't provide one manually Sigil generates it for you using the template "sigil_toc_id_#". In your case the link from the .ncx would be:
"<content src="Text/{your sheet name}.xhtml#sigil_toc_id_75"/>
and from an html file would be:
<a href="{your sheet name}.xhtml#sigil_toc_id_75">
You could change them to something else if you really want to, but the user will never see them and they are OK.