What does adjusted cost base mean related to sale of the sale of my principal res? Is it purchase price less the legal fees to sell it and cost of breaking mortgage?

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As per CRA, 

The adjusted cost base (ACB) is usually the cost of a property plus any expenses to acquire it, such as commissions and legal fees.

You can calculate Capital gain as Proceeds from disposition- Adjusted cost base (ACB- usually the cost of a property plus any expenses to acquire it, such as commissions and legal fees)-Outlays and Expenses (Amounts that you incurred to sell a capital property).  

Outlays and Expenses (Amounts that you incurred to sell a capital property)-The cost of breaking a mortgage would be part of the 'outlays and expenses' when selling a property.

Please visit the article on Reporting the Sale of Your Principal Residence and How do I report the sale of my home (principal residence)?