What does it mean by "many different transactions tied to a particular bitcoin address" in this context? The context is on this section: “What if the input and output amounts don’t match?” of this page: http://www.coindesk.com/information/how-do-bitcoin-transactions-work/

I find it strange as it seems that reusing address is a bad practice (https://en.bitcoin.it/wiki/Address_reuse). (i.e. When Person A makes a transaction with Person B, then Person A makes a transaction with Person C, there should be two different addresses (instead of one) from Person A involved.) Or does the “address” here actually mean bitcoin wallet?
many different transactions tied to a particular bitcoin address


The sentence refers to the fact that the address can be involved in many different transactions. This is most likely seen with the address working as a receiving address, or destination address. For instance, imagine that you want to have an address posted in your personal website where you receive donations from people. It would be tedious to change it every time you receive a transaction. 

As you already mentioned, reusing an address is not usually a good practice, since it could involve some privacy issues. By using a different address each time you perform a payment, you make it harder to link different bitcoin addresses owned by you (however, not impossible). By using always the same address when paying you make way easier for a third party to analyse your behaviour, based on where you are sending bitcoins. 

Finally you should not mix up Bitcoin address with wallet. Think in a wallet as a collection of tools that let you manage your Bitcoin address (or addresses), such managing public/private keys to create transactions. What is said to be a bad practice in the Bitcoin wiki is referring to use the same Bitcoin address multiple times. The wallet is a collection of tools, the Bitcoin address will be more like an endpoint.