der Sechser
six
The digit or number 6.
Beim Würfeln brauche ich einen Sechser, um zu gewinnen.
When rolling the dice, I need a six to win.

six / defensive midfielder / six-pack
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six
The digit or number 6.
Beim Würfeln brauche ich einen Sechser, um zu gewinnen.
When rolling the dice, I need a six to win.

The word is derived from the number 'sechs' (six). The ending '-er' turns the number into a noun that refers to the number itself or to something connected with it.
Er hat eine Telefonnummer mit einem Sechser am Ende.
He has a phone number with a six at the end.
You can recognize the many meanings of 'Sechser' from the topic. In sports, it refers to the soccer player; when shopping, to the drink pack; at school, to the grade; and in gambling, to the lottery win.
Der Trainer lobte seinen Sechser für die gute Abwehrarbeit.
The coach praised his defensive midfielder for the good defensive work.
In soccer jargon, 'der Sechser' is a very common term for the defensive midfielder. People also speak of the 'Sechser-Position' (number-six position).
Er spielt auf der Position des Sechsers.
He plays in the defensive midfielder position.
In this meaning, 'Sechser' is usually combined with a type of drink. Typical examples are 'ein Sechser Bier' (a six-pack of beer), 'ein Sechser Cola' (a six-pack of cola), or 'ein Sechser Wasser' (a six-pack of water).
Kannst du vom Supermarkt einen Sechser Limonade mitbringen?
Can you bring a six-pack of lemonade from the supermarket?
This meaning is only understandable in the context of the German school system. The grading scale ranges from 1 (very good) to 6 (insufficient). A 'Sechser' is therefore the worst possible grade.
Für die nicht abgegebene Hausaufgabe hat er einen Sechser bekommen.
He got a failing grade for the homework he didn't hand in.
The expression 'ein Sechser im Lotto' is very often used metaphorically for an extreme stroke of luck or a direct hit. It does not have to be about real money.
Dieser Job in dieser schönen Stadt ist für mich wie ein Sechser im Lotto.
This job in this beautiful city is like winning the lottery jackpot for me.
six
'sechs' and 'der Sechser' both have to do with the number 6. 'sechs' is the normal number: you use it when counting, for quantities, or also for the school grade. 'der Sechser' is a noun and often means the digit 6 or a grade of 6. Learners may confuse the words because both refer to the same number. As a practical rule: if you are simply naming a quantity, you say 'sechs'. If you are talking about the digit or the grade as a thing, 'der Sechser' fits.

Auf dem Zeugnis war ein Sechser in Mathe.
There was a failing grade in math on the report card.
Für das Rezept brauchen wir sechs Eier.
For the recipe, we need six eggs.