As per the fatwa of Ayatolaah sistani, purchasing products that support israel are haram. What is meant by 'support' ? Does it mean that any company that is selling its products in Israel, has to be compulsorily boycotted and its products become haram for us to purchase?

It means any company that supports the state of Israel and its oppressive practices. So if a company is known to donate money to the government of Israel, or it funds its unjust policies, or it empowers the state financially, then purchasing products from it is haram.

Now if a company simply sells its products in Israel, then it doesn’t necessarily mean that it is supporting the state of Israel. In that case it would be fine to buy from this company.