In Systemic Functional Linguistics, the term strata refers to the hierarchical organization of language as a social semiotic system. In simpler words, the term describes how the social context influences the way we use language to express meanings. The featured image above shows the hierarchy.
Context
This is the overarching element, the context. In SFL, it specifically refers to the context of situation or register. Think of a situation in which the interactants are socially distant [tenor] (e.g., between a teacher and a student). Typically, the student will use words and structure differently compared to a situation where he speaks with his classmate. This is particularly true when the topic under discussion relates to the subject [field]. The same interactants may use different words and structures (lexicogrammar) when talking about personal experiences. Also, they will communicate differently in writing (e.g., text messages), compared to speaking (e.g., face-to-face mini conference or via telephone) [mood].
Discourse Semantic
The context above provides choices for words and structures to select to express meaning. This is not confined to the words' meaning, but what they want to convey in general. For instance, the communication may be a simple self-introduction or an intense debate. The former is to describe and inform, while the latter is to argue. In either case, there must be ideas or experiences to say [ideational] and attempts to engage with each other [interpersonal]. Some words or phrases may also be used to link or sequence messages [textual].
Lexicogrammar
Lexicogrammar is a broad term in SFL, but in this context, it refers to the linguistic choices made by the interactants, considering the context and the meaning or discourse semantics. Lexicogrammar is a combination of the words lexes (vocabulary) and grammar (structure). SFL argues that we use words and structures by choosing them with the context and discourse semantics taken into consideration.
Phonology/Graphology
This is the end, the final result. Phonology is concerned with oral communication (spoken text), while graphology is concerned with written communication (written text). However, graphology can also include the use of figures or tables; it is called multimodal text.
Conclusion
The concept of strata in SFL shows that our linguistic choices are always contextual. We use language not by following "rules," but by considering the context of the situation. Of course, we have meanings to convey, but there are many ways to express one meaning. This concept explains how we choose one pattern over the others.