Okay, so modus tenendi parliamentum is a fancy Latin phrase that means "the way of holding Parliament".
Do you know what Parliament is? It's a group of important people who make rules and decisions for a country. In some countries, like England, there's a special building called the House of Parliament where these important people meet.
Now, modus tenendi parliamentum refers specifically to the way that Parliament is organized and run. It's like the rules for playing a game - everyone needs to follow them for things to work properly.
Back in the olden days, when people wanted to have a Parliament meeting, they would first send out special invitations to all the important people who needed to come. Then, they would choose a special place for everyone to meet - usually a big room in a castle or fancy house.
At the actual meeting, there were lots of rules that everyone had to follow. For example, there was a special order for speaking - you couldn't just talk whenever you wanted, you had to wait your turn. And if someone interrupted you while you were speaking, that was considered very rude.
Overall, modus tenendi parliamentum is just a fancy way of saying that there are rules for how Parliament works, and everyone needs to follow them in order to have a successful meeting.