When muslims do any deed an bismillah is uttered ie in the name of Allah. What it means could differ in context. Literally it means in the name of Allah. What it means could be in seek permission of Allah, I am doing for sake of Allah, I do this because it is permitted by Allah.
When an animal is slaughtered for it to be permitted(Halal) for consumption by a muslim a few criteria have to be met. 1) it should be among the permitted animal types. 2) It should be an healthy animal. 3) pertaining to the second point an animal which is pregnant, which has been starving, which has been transported thus not had access to food (some times animals eat their own extrement due to hunger) needs to be first relaxed and healthy. In case of transported animals feeding of normal food for 3 days is recommended. 4) Animal should not be at any time consuming the food made out of meat of any animal including its own kind.
Then comes the criteria of slaughtering.
These constitute Slaughtering by cutting the wind pipe and the 3 main arteries in one to one and half knife stroke(Not as many sikhs allege by butchering the throat) The spinal chord has to remain intact to facilitate the body to pump out the blood which is prohibited(Haram).
Before slaughtering the animal Bismillah is uttered and some times Allah hu Akbar is uttered the Idea here is a muslim is not permitted to use or kill animals except in certain circumstances. For instance use of animals for riding, milk , agricultural work as beasts of burden etc is permitted. Use for pleasure by confining animal like pet keeping is frowned. Killing animals for sake of pleasure is prohibited. For purpose of food or safety hunting is permitted.
It is in this context that the Bismillah is used ie I take this life because it has been permitted for food. Muslims loosely describe this as a blessing some others describe this as an offering of sacrifice. Both are far from the truth. Here what is happening is a muslim is acting as per the permisson granted to him to use the meat of an animal and is reinforcing this permission by uttering out the words in the name of Allah ie in the name of the one who has made it permissible to take this life for sake of food.
Muslims even utter Bismillah while entering an house, leaving for their job, or even drinking water it does not mean that they are sacrificing any of those things for Allah or are offering it to Allah.