estate = the person's property. For example: house, land, car...
estate planning = deciding who gets what property and when.
For example: When they marry, the person wants their partner to own half of the house.
The person wants to give their house to one of their children after they die.
The estate plan will probably be an official document that states who owns (or will own) what property.
tax consequences = depending on marital status, you can pay or claim different amounts of tax. If the person gets married, that will change the amount of tax that they have to pay.
In this example we are talking about "legal benefits". Benefits usually mean something good. So, by marrying a same-sex partner, you would probably pay less tax.