In sutta SN 40.9 and SN 41.7,what do they mean by "signless" immersion of the heart?

I'm still learning the Suttas to apply The Buddha's teachings to my life. I haven't read much about the jhanas (which I think the below is related to) so I ask. 1. What does it mean when The Buddha says "signless of the heart?" 2. How does this relate to nothingness or immersion of the heart? 3. What are "signs?" Thank you. SN 40.9 “They speak of this thing called the ‘signless immersion of the heart’. What is the signless immersion of the heart? It occurred to me: ‘It’s when a mendicant, not focusing on any signs, enters and remains in the signless immersion of the heart. This is called the signless immersion of the heart.’ (Excerpt) https://www.accesstoinsight.org/tipitaka/sn/sn40/sn40.009.wlsh.html 41.7 And what is the release of the heart through emptiness? It’s when a mendicant has gone to a wilderness, or to the root of a tree, or to an empty hut, and reflects like this: ‘This is empty of a self or what belongs to a self.’ This is called the release of the heart through emptiness. And what is the signless heart’s release? It’s when a mendicant, not focusing on any signs, enters and remains in the signless immersion of the heart. This is called the signless release of the heart.(Excerpt) https://suttacentral.net/sn41.7/en/sujato?layout=plain&reference=none¬es=asterisk&highlight=false&script=latin