The content needs to be comprehensible. Let me explain using one of the four skills -- reading.

In reading, understanding less than 90% of a text is "pain reading", 90%-98% of a text is intensive reading, and 98%-100% is extensive reading. Students need intensive reading and extensive reading. "Pain reading" is a poor use of time. It ends with the student hating reading and hating learning English.

Intensive reading would be like Krashen's input + 1 (i+1) or Vygotsky's zone of proximal development (ZPD). The basic idea is the same -- scaffolding. Students should read texts that are little more difficult than they can understand for intensive reading. However, they should be able to construct meaning with teacher support and contextual clues. If the student can't understand more than 10% of the text, it's too difficult.

Extensive reading is also important. Students need to read 5,000,000 words to into a good college (Waring, 2017). Students need to read lots of texts that are fast and fluent. This builds vocabulary, allows students to meet grammar and vocabulary structures over and over, and provides an authentic context for learning. If you want to read more about the topic of reading and scaffolding, I'd suggest the articles written by Rob Waring.