He meant that you can't trust your 5 senses to know that what you see/hear etc is real, and if you are really here right now. Your senses can mislead you in how the world looks (eg colour blindness)...
If you can think, you must be. on the other hand, if you are, that doesn't necessarily mean you can think. so you think, therefore you are.
One who thinks is conscious of his own existence. Tree doesn't think (as we know) and it isn't a conscious being, a person.
francis bacon studied literature and wrote many books on the subjects of science and philosophy.